For safety and economic reasons, aircraft must have the capability of determining accurately the amount of fuel onboard. Thus, various fuel gauging systems and other sensing apparatus have been utilized to deliver information concerning the status of fuel carried by the aircraft to the pilot. In general, all these systems require portions of electronic circuits, such as, wiring, probes, sensors, etc., to extend inside the fuel tank.
Examples of known aircraft fuel gauging systems are disclosed in U.S. Pat. No. 5,602,333 issued to Larrabee et al.; U.S. Pat. No. 4,918,619 issued to Orloff et al., U.S. Pat. No. 4,908,783 issued to Maier; U.S. Pat. No. 4,872,120 issued to Orloff et al.; U.S. Pat. No. 4,731,730 issued to Hedrick et al.; U.S. Pat. No. 4,451,894 issued to Dougherty et al.; U.S. Pat. No. 4,487,066 issued to Pardi et al.; U.S. Pat. No. 4,173,893 issued to Hedrick; and U.S. Pat. No. 4,090,408 issued to Hedrick.
Permitting excessive current to flow in wiring or electronic components located inside a fuel tank creates a potentially dangerous situation. Thus, the amount of current permitted in electronics in fuel tanks must be limited during normal operations of the systems and during unwanted fault conditions.
One example of a typical fault condition is a ground fault which results in a decrease in circuit resistance which, in turn, causes an increase In current. Another possible fault condition can occur between wiring of various circuits which are bundled together at locations exterior of the fuel tank along a given length of cableway. Such faults can result in greater voltages being applied on circuits extending into the fuel tank which, in turn, causes an increase in current in the electronics in the fuel tank.
Various surge protectors, circuit breakers and other current limiting devices are known which can be used to limit current through a circuit or a portion thereof For example, in the case of an aircraft fuel gauging system, transient suppressors can be electrically connected on each line entering or leaving the fuel tank to limit the voltage level, and a series resistor can be included on each line to set the resulting maximum current. However, since stray capacitance exists within an aircraft fuel tank, the addition of the series resistor causes a phase shift of the signal carried by the circuit, thereby degrading the accuracy of the fuel gauging system during normal operation.
Therefore, although various current limiting devices are known in the art which may be satisfactory for their intended purposes, there is a need for a current limiting device particularly useful in limiting the current through electronics located in and extending through the fuel tank of an aircraft. The device should be capable of limiting current without degrading the accuracy of the fuel gauging system.